A conventional laundering machine employs a constant-speed motor to drive an agitator and a coaxially arranged laundry basket. The agitator rotates in a back-and-forth motion for agitating fluid and fabrics in the laundry basket during an agitate mode, the agitator oscillation being relatively slow compared with the motor speed. Rotation of the basket during a spin mode for effecting centrifugal displacement of fluid and particulate matter from the fabrics, however, is unidirectional at a speed appreciably greater than that of the agitator oscillation. Thus, a complex transmission is required to adapt the constant-speed motor to operate the laundering machine in both the agitate and spin modes. As a further disadvantage, there is a large volume requirement for housing such motor and transmission systems.
To overcome the aforementioned shortcomings, adjustable speed drives comprising electronically commutated motors (ECMs) for use in laundering machines have been developed. One such device is disclosed in U.S. Pat. No. 4,556,827 issued Dec. 3, 1985 to David M. Erdman and assigned to the instant assignee. The laundering apparatus of the cited patent includes an electronically commutated motor comprising: a stationary assembly; a multistage winding arrangement associated with the stationary assembly and having a plurality of winding stages adapted to be commutated in a plurality of preselected sequences; and rotatable means rotatably associated with the stationary assembly and arranged in selective magnetic coupling relation with the winding stages for driving the apparatus. Although the ECMs advantageously reduce the complexity, size and cost of laundering apparatus, it is desirable to reduce these factors even further. The present invention, therefore, utilizes a switched reluctance motor (SRM) drive system for operating laundering apparatus. In contrast to an ECM, a SRM requires no permanent magnets and no rotor windings. Hence, a SRM is both simple and economical in construction.
Switched reluctance motors conventionally have multiple poles on both the stator and the rotor; that is, they are doubly salient. There are phase windings on the stator, but no windings or magnets on the rotor. Each pair of diametrically opposite stator pole windings is connected in series to form an independent phase winding of the multiphase switched reluctance motor. Torque is produced by switching current in each phase winding in a predetermined sequence that is synchronized with angular position of the rotor, so that a magnetic force of attraction results between the rotor poles and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow, so that unidirectional current pulses synchronized with rotor movement can be applied to the stator phase windings by a converter using unidirectional current switching elements, such as thyristors or transistors.
A SRM drive operates by switching the stator phase currents on and off in synchronism with rotor position. By properly positioning the firing pulses relative to rotor angle, forward or reverse operation and motoring or generating operation can be obtained. Usually, the desired phase current commutation is achieved by feeding back a rotor position signal to a controller from a shaft position sensor, e.g. an encoder or a resolver. To improve reliability and to reduce size, weight, inertia, and cost in such drives, it is desirable to eliminate this shaft position sensor. To this end, various approaches have been previously proposed for indirect rotor position sensing by monitoring terminal voltages and currents of the motor. One such approach, referred to as waveform detection, depends upon back electromotive forces (EMF) and is, therefore, unreliable at low speeds and inoperative at zero speed. Such an approach would not be suitable for use in high-torque, low-speed laundering machines.
Another approach to indirect rotor position sensing is disclosed in commonly assigned U.S. Pat. No. 4,772,839, issued Sept. 20, 1988 to S. R. MacMinn and P. B. Roemer, which is incorporated by reference herein. The cited patent discloses an indirect position estimator for a SRM which applies low level sensing pulses of short duration to the unenergized motor phases. Application of the sensing pulses results in a change in current in each of the unenergized phases. The change in current is sensed by a current sensor and an estimated inductance value is derived therefrom. A pair of estimated rotor angles corresponding to the estimated inductance value for each of the unenergized phases is ascertained. One such pair is shifted by a value equal to a known phase displacement of the other unenergized phase. The pairs of estimated angles are then compared to determine which of the angles match. An estimated instantaneous rotor angular position equal to the matching angle is produced. Moreover, in case any of the stator phases undergoes a change in state during sampling or in case two phases do not remain energized throughout the sampling, an extrapolator is provided to generate an extrapolated rotor angular position instead of the estimated position.
Although the above-cited patent advantageously provides a method for indirectly determining rotor position so that a conventional rotor position sensor is not required, it is desirable to provide an even simpler method and associated apparatus. Further, it is desirable to eliminate the need for discrete current sensing devices which likewise add to the complexity, size, weight and cost of a SRM drive system. Such a "sensorless" system would improve and enhance the operational characteristics of SRMs and extend their applicability to, for example, consumer appliances.